[musl] [PATCH 1/1] IN6_IS_ADDR_LOOPBACK() and similar macros warn on -Wcast-qual

[Brad House <brad@brad-house.com>]

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I'm the maintainer of c-ares (https://c-ares.org) and have been scanning 
the CI build logs for various systems to catch warnings, and on Alpine 
Linux (which obviously uses musl c) we get these warnings, specifically 
when using clang (but not oddly not on gcc):

/__w/c-ares/c-ares/src/lib/ares__sortaddrinfo.c:93:9: warning: cast from 
'const struct in6_addr *' to 'unsigned char *' drops const qualifier 
[-Wcast-qual]
    93 |     if (IN6_IS_ADDR_MULTICAST(&addr6->sin6_addr)) {
       |         ^
/usr/include/netinet/in.h:120:48: note: expanded from macro 
'IN6_IS_ADDR_MULTICAST'
   120 | #define IN6_IS_ADDR_MULTICAST(a) (((uint8_t *) (a))[0] == 0xff)
       |                                                ^
/__w/c-ares/c-ares/src/lib/ares__sortaddrinfo.c:95:16: warning: cast 
from 'const struct in6_addr *' to 'unsigned int *' drops const qualifier 
[-Wcast-qual]
    95 |     } else if (IN6_IS_ADDR_LOOPBACK(&addr6->sin6_addr) ||
       |                ^
/usr/include/netinet/in.h:115:24: note: expanded from macro 
'IN6_IS_ADDR_LOOPBACK'
   115 |         (((uint32_t *) (a))[0] == 0 && ((uint32_t *) (a))[1] == 
0 && \
       |                        ^
/__w/c-ares/c-ares/src/lib/ares__sortaddrinfo.c:96:16: warning: cast 
from 'const struct in6_addr *' to 'unsigned char *' drops const 
qualifier [-Wcast-qual]
    96 | IN6_IS_ADDR_LINKLOCAL(&addr6->sin6_addr)) {
       |                ^
/usr/include/netinet/in.h:123:24: note: expanded from macro 
'IN6_IS_ADDR_LINKLOCAL'
   123 |         ((((uint8_t *) (a))[0]) == 0xfe && (((uint8_t *) 
(a))[1] & 0xc0) == 0x80)
       |                        ^
/__w/c-ares/c-ares/src/lib/ares__sortaddrinfo.c:141:16: warning: cast 
from 'const struct in6_addr *' to 'unsigned int *' drops const qualifier 
[-Wcast-qual]
   141 |     } else if (IN6_IS_ADDR_V4MAPPED(&addr6->sin6_addr)) {
       |                ^
/usr/include/netinet/in.h:129:24: note: expanded from macro 
'IN6_IS_ADDR_V4MAPPED'
   129 |         (((uint32_t *) (a))[0] == 0 && ((uint32_t *) (a))[1] == 
0 && \
       |                        ^
/__w/c-ares/c-ares/src/lib/ares__sortaddrinfo.c:149:16: warning: cast 
from 'const struct in6_addr *' to 'unsigned int *' drops const qualifier 
[-Wcast-qual]
   149 |     } else if (IN6_IS_ADDR_V4COMPAT(&addr6->sin6_addr)) {
       |                ^
/usr/include/netinet/in.h:134:24: note: expanded from macro 
'IN6_IS_ADDR_V4COMPAT'
   134 |         (((uint32_t *) (a))[0] == 0 && ((uint32_t *) (a))[1] == 
0 && \
       |                        ^
/__w/c-ares/c-ares/src/lib/ares__sortaddrinfo.c:151:16: warning: cast 
from 'const struct in6_addr *' to 'unsigned char *' drops const 
qualifier [-Wcast-qual]
   151 |     } else if (IN6_IS_ADDR_SITELOCAL(&addr6->sin6_addr)) {
       |                ^
/usr/include/netinet/in.h:126:24: note: expanded from macro 
'IN6_IS_ADDR_SITELOCAL'
   126 |         ((((uint8_t *) (a))[0]) == 0xfe && (((uint8_t *) 
(a))[1] & 0xc0) == 0xc0)
       |                        ^
/__w/c-ares/c-ares/src/lib/ares__sortaddrinfo.c:189:16: warning: cast 
from 'const struct in6_addr *' to 'unsigned int *' drops const qualifier 
[-Wcast-qual]
   189 |     } else if (IN6_IS_ADDR_V4COMPAT(&addr6->sin6_addr) ||
       |                ^
/usr/include/netinet/in.h:134:24: note: expanded from macro 
'IN6_IS_ADDR_V4COMPAT'
   134 |         (((uint32_t *) (a))[0] == 0 && ((uint32_t *) (a))[1] == 
0 && \
       |                        ^

Full build output: 
https://github.com/c-ares/c-ares/actions/runs/10219723015/job/28278549865

I've attached a patch that will silence this warning by always casting 
to the comparison to const, but otherwise not impact the behavior.

-Brad

[-- Attachment #2: musl-netinet-warning-fix.patch --]
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diff --git a/include/netinet/in.h b/include/netinet/in.h
index fb628b61..f04657f3 100644
--- a/include/netinet/in.h
+++ b/include/netinet/in.h
@@ -108,46 +108,63 @@ uint16_t ntohs(uint16_t);
 #define IPPROTO_MAX      263
 
 #define IN6_IS_ADDR_UNSPECIFIED(a) \
-        (((uint32_t *) (a))[0] == 0 && ((uint32_t *) (a))[1] == 0 && \
-         ((uint32_t *) (a))[2] == 0 && ((uint32_t *) (a))[3] == 0)
+        (((const uint32_t *) (a))[0] == 0 && \
+         ((const uint32_t *) (a))[1] == 0 && \
+         ((const uint32_t *) (a))[2] == 0 && \
+         ((const uint32_t *) (a))[3] == 0)
 
 #define IN6_IS_ADDR_LOOPBACK(a) \
-        (((uint32_t *) (a))[0] == 0 && ((uint32_t *) (a))[1] == 0 && \
-         ((uint32_t *) (a))[2] == 0 && \
-         ((uint8_t *) (a))[12] == 0 && ((uint8_t *) (a))[13] == 0 && \
-         ((uint8_t *) (a))[14] == 0 && ((uint8_t *) (a))[15] == 1 )
+        (((const uint32_t *) (a))[0] == 0 && \
+         ((const uint32_t *) (a))[1] == 0 && \
+         ((const uint32_t *) (a))[2] == 0 && \
+         ((const uint8_t *) (a))[12] == 0 && \
+         ((const uint8_t *) (a))[13] == 0 && \
+         ((const uint8_t *) (a))[14] == 0 && \
+         ((const uint8_t *) (a))[15] == 1 )
 
-#define IN6_IS_ADDR_MULTICAST(a) (((uint8_t *) (a))[0] == 0xff)
+#define IN6_IS_ADDR_MULTICAST(a) (((const uint8_t *) (a))[0] == 0xff)
 
 #define IN6_IS_ADDR_LINKLOCAL(a) \
-        ((((uint8_t *) (a))[0]) == 0xfe && (((uint8_t *) (a))[1] & 0xc0) == 0x80)
+        ((((const uint8_t *) (a))[0]) == 0xfe && \
+         (((const uint8_t *) (a))[1] & 0xc0) == 0x80)
 
 #define IN6_IS_ADDR_SITELOCAL(a) \
-        ((((uint8_t *) (a))[0]) == 0xfe && (((uint8_t *) (a))[1] & 0xc0) == 0xc0)
+        ((((const uint8_t *) (a))[0]) == 0xfe && \
+         (((const uint8_t *) (a))[1] & 0xc0) == 0xc0)
 
 #define IN6_IS_ADDR_V4MAPPED(a) \
-        (((uint32_t *) (a))[0] == 0 && ((uint32_t *) (a))[1] == 0 && \
-         ((uint8_t *) (a))[8] == 0 && ((uint8_t *) (a))[9] == 0 && \
-         ((uint8_t *) (a))[10] == 0xff && ((uint8_t *) (a))[11] == 0xff)
+        (((const uint32_t *) (a))[0] == 0 && \
+         ((const uint32_t *) (a))[1] == 0 && \
+         ((const uint8_t *) (a))[8] == 0 && \
+         ((const uint8_t *) (a))[9] == 0 && \
+         ((const uint8_t *) (a))[10] == 0xff && \
+         ((const uint8_t *) (a))[11] == 0xff)
 
 #define IN6_IS_ADDR_V4COMPAT(a) \
-        (((uint32_t *) (a))[0] == 0 && ((uint32_t *) (a))[1] == 0 && \
-         ((uint32_t *) (a))[2] == 0 && ((uint8_t *) (a))[15] > 1)
+        (((const uint32_t *) (a))[0] == 0 && \
+         ((const uint32_t *) (a))[1] == 0 && \
+         ((const uint32_t *) (a))[2] == 0 && \
+         ((const uint8_t *) (a))[15] > 1)
 
 #define IN6_IS_ADDR_MC_NODELOCAL(a) \
-        (IN6_IS_ADDR_MULTICAST(a) && ((((uint8_t *) (a))[1] & 0xf) == 0x1))
+        (IN6_IS_ADDR_MULTICAST(a) && \
+         ((((const uint8_t *) (a))[1] & 0xf) == 0x1))
 
 #define IN6_IS_ADDR_MC_LINKLOCAL(a) \
-        (IN6_IS_ADDR_MULTICAST(a) && ((((uint8_t *) (a))[1] & 0xf) == 0x2))
+        (IN6_IS_ADDR_MULTICAST(a) && \
+         ((((const uint8_t *) (a))[1] & 0xf) == 0x2))
 
 #define IN6_IS_ADDR_MC_SITELOCAL(a) \
-        (IN6_IS_ADDR_MULTICAST(a) && ((((uint8_t *) (a))[1] & 0xf) == 0x5))
+        (IN6_IS_ADDR_MULTICAST(a) && \
+         ((((const uint8_t *) (a))[1] & 0xf) == 0x5))
 
 #define IN6_IS_ADDR_MC_ORGLOCAL(a) \
-        (IN6_IS_ADDR_MULTICAST(a) && ((((uint8_t *) (a))[1] & 0xf) == 0x8))
+        (IN6_IS_ADDR_MULTICAST(a) && \
+         ((((const uint8_t *) (a))[1] & 0xf) == 0x8))
 
 #define IN6_IS_ADDR_MC_GLOBAL(a) \
-        (IN6_IS_ADDR_MULTICAST(a) && ((((uint8_t *) (a))[1] & 0xf) == 0xe))
+        (IN6_IS_ADDR_MULTICAST(a) && \
+         ((((const uint8_t *) (a))[1] & 0xf) == 0xe))
 
 #define __ARE_4_EQUAL(a,b) \
 	(!( (0[a]-0[b]) | (1[a]-1[b]) | (2[a]-2[b]) | (3[a]-3[b]) ))